1. Field of the Invention
The present invention relates to a downshift control device for an automatic transmission, and more particularly to a technique for controlling a pressure applied to an engaging element and a pressure released from another engaging element in the process of shifting down of the transmission which is initiated by depressing an accelerator pedal or manually selecting a gear position while the vehicle is in a power-on condition.
2. Description of the Prior Art
There is known a shift control device for an automatic transmission as disclosed in Japanese laid-open Patent Publication No. 2-80850. In order to reduce shift shocks when the transmission is shifted down in a power-on condition of the vehicle, the control device disclosed in this publication is provided with delaying means for delaying engagement of a first input clutch that is to be engaged upon downshifting (by restricting or reducing an increase in the pressure applied to the first input clutch in the later period of shifting), so as to prevent the first input clutch from being engaged with a transmission torque capacity before a first one-way clutch is engaged such that the rotating speed of an input shaft of the transmission matches the gear position in which the transmission is to be placed after the downshifting.
Since the above-described known downshift control device employs the one-way clutch for optimizing the downshift timing in which the input and output shafts of the transmission are rotated in synchronization, pull-in or drawing shocks can be avoided and desirable downshifting can be thus achieved by suitably controlling the pressure applied to the clutch during downshifting and restricting the increase in the rotating speed of the input shaft of the transmission. In an automatic transmission equipped with a power transmitting system in which the one-way clutch is eliminated, on the other hand, the downshift timing for realizing the synchronized rotation is determined by monitoring the increase in the pressure applied to the clutch, but the rotating speed of the input shaft of the transmission cannot be controlled by controlling the pressure applied to the clutch.
In the automatic transmission equipped with the power transmitting system in which the one-way clutch is eliminated, therefore, when the clutch pressure to be applied is increased earlier than the synchronization timing (as indicated in "k" in FIG. 14), by keeping the engine speed NE (turbine speed NT) being increased at a high rate (steep slope of increase in the engine speed NE) throughout the downshift operation, as shown in FIG. 14, the rotation of the input shaft is pulled up and the output torque TQ becomes temporarily negative, resulting in pull-in shocks. These shocks may be reduced where the clutch pressure to be applied is increased concurrently with the synchronization timing (as indicated in "m" in FIG. 14). Where the clutch pressure to be applied is increased later than the synchronization timing (as indicated in "n" in FIG. 14), however, the rotation of the engine that is racing is pulled down, and the output torque TQ is temporarily raised to a great extent, resulting in thrust shocks.
As described above, where the slope of increase in the engine speed NE (turbine speed NT) (=slope of increase in the gear ratio) greatly changes in a region around the synchronization timing, the clutch pressure to be applied must be increased in the timing that is almost equivalent to a point of time or a moment. Where rotating conditions, oil temperature and other conditions slightly differ each time the transmission is shifted down, therefore, the clutch pressure to be applied cannot be always increased at the same time as the synchronous rotation of the input and output shafts of the transmission, and the pull-in shocks or thrust shocks cannot be avoided.